1. Field of the Invention
The present invention relates to zirconium oxide or a precursor thereof which can be used as an electrolyte and an electrode of fuel cells, an oxygen sensor, an oxygen-enriched membrane, a heating element, a substrate of biosensor, bioreactors, etc., a catalyst or a carrier of catalysts, a catalyst for removing or decomposing NOx of exhaust from internal combustion engines (e.g., gasoline engines and diesel engines), boilers and industrial plants, a support of artificial bone, heat-resistant materials, and the like. It also relates to a process for producing them.
2. Description of the Related Art
Zirconium oxide (zirconia), solid solutions of zirconium oxide and other oxides, zirconium oxide crystals, and porous bodies composed of these substances are highly resistant against heat and corrosion and exhibit high oxygen ion conductivity and high electron conductivity and have therefore been used as an electrolyte and an electrode of fuel cells, an oxygen sensor, an oxygen-enriched membrane, a heating element, a substrate of biosensor, bioreactors, etc., a catalyst or a carrier of catalysts, a support of artificial bone, heat-resistant materials, and the like.
In these applications, it has been demanded to establish a convenient technique for forming a thin and uniform crystal film on various substrates or, for some uses, dispersively forming a crystal precipitate on substrates for the purpose of reducing the distance of electron or ion movement thereby to increase electric conductivity or sensitivity; increasing gas permeability; securing a large surface area or contact area; or reducing the weight.
Methods of forming a thin metal oxide film include CVD, ion plating, sputtering and the like. However, these methods require special expensive equipment and meet difficulty in forming a wide thin film or a thin film on a substrate having a complicated surface profile, particularly in forming a thin film for a large-sized fuel cell.
Another method for forming a metal oxide film comprises coating a substrate with a dispersion prepared by mixing a powdered metal oxide, a binder, and a dispersant, followed by drying. With this method, however, it is difficult to form a sufficiently thin and yet strong film.
Being light and resistant to abrupt temperature changes, zirconium oxide is a promising material of various furnaces and machine parts. These parts have been manufactured by mixing powder mainly comprising microcrystalline zirconium oxide with a forming aid, a dispersant, etc., forming the mixture into a green body, and heat-treating the green body in high temperature. As reported, e.g., in J. Phy. Chem., vol. 69, p. 1238 (1965), zirconium oxide is stable in its monoclinic phase in a temperature range of from room temperature to about 1100xc2x0 C. and in its tetragonal or cubic phase at higher temperatures. The phase transition from the monoclinic phase to the tetragonal or cubic phase or vice versa is reversible and is accompanied with a volumetric change. That is, the phase transition repeatedly takes place through the heat treatment after forming and temperature rises and drops in practical use, which results in embrittlement of the formed body.
This can be prevented by incorporating several mole percents of yttrium oxide into zirconium oxide to restrain zirconium oxide in the high-temperature phase (i.e., the tetragonal or cubic phase). It is a generally followed practice to use yttria-stabilized zirconium oxide as a forming material. However, addition of yttrium oxide, which is expensive, increases the cost of the resulting formed body.
There is a complicated relationship between the crystal grain size and the crystal structure of pure zirconium oxide as described, e.g., in J. European Ceram. Soc., vol. 19, p. 1827 (1999). It is not easy to control the relationship. Further, the method of crystal growth control, which is also complicated, is influenced by the starting compound and trace impurities present therein (see, for example, J. Am. Ceram. Soc., vol. 49, p. 286 (1966)) and usually requires accurate temperature control. Moreover, crystallization takes a long time.
On the other hand, Chem. Lett., p. 575 (1999) reports that a zirconium oxide catalyst for selectively synthesizing isobutene from a carbon monoxide/hydrogen mixed gas has its selectivity perfectly correlated to the content of monoclinic crystals. Phys. Chem. Chem. Phys., vol. 1, p. 2825 (1999) teaches that an acid catalyst comprising a zirconium oxide carrier having sulfate ions supported thereon shows correlation between the amount of supported sulfate ions and the surface structure of the carrier that is decided by the crystal form. It is therefore conceivable that the function as an acid catalyst varies depending on the crystal form. Accordingly, in order to efficiently design a high-performance catalyst, a method for selectively obtaining zirconium oxide of desired crystal form has been sought.
An object of the present invention is to provide a process for produce thin film of zirconium oxide and a process for efficiently produce zirconium oxide having a specific crystal structure and zirconium oxide crystals obtained by the process.
Another object of the present invention is to provide a process for producing zirconium oxide or a precursor thereof, by which a thin and dense film comprising small particles of zirconium oxide or a precursor thereof can easily be formed on various substrates including organic substances, large-area substrates, or substrates having a complicated surface profile such as a porous material by means of simple equipment and without requiring a heating step for crystallization for stabilization; and zirconium oxide or a precursor thereof prepared by the process.
Still another object of the present invention is to provide zirconium oxide or a precursor thereof which is capable of effectively reduce or decompose nitrogen oxides (NOx) contained in exhaust, etc. and which can easily be produced.
The present inventors have conducted extensive investigation to accomplish the above objects. As a result, they have found that desired zirconium oxide or a precursor thereof can be obtained from an alcohol solution of a zirconium compound through simple operations. They have also found that the resulting zirconium oxide has characteristic peaks assigned to a specific Sxe2x80x94O bond in the infrared absorption spectrum and that the peaks are different from those assigned to the Sxe2x80x94O bond of commercially available sulfated zirconia.
Based on the above findings, the invention provides:
1) Zirconium oxide having infrared absorption peaks between 1010 cmxe2x88x921 and 1025 cmxe2x88x921 and between 1035 cmxe2x88x921 and 1050 cmxe2x88x921 which are assigned to an Sxe2x80x94O bond.
2) A process for producing a zirconium oxide precursor comprising dissolving a zirconium compound in an alcohol, immersing a substrate in the solution, and precipitating a zirconium oxide precursor on the substrate, a zirconium oxide precursor composite obtained by the process, and a catalyst for removing nitrogen oxides which comprises the zirconium oxide precursor.
3) A process for producing a zirconium oxide precursor comprising dissolving a zirconium compound in an alcohol and allowing the zirconium compound to react at a temperature of from 0xc2x0 C. up to the boiling point of the system to precipitate a zirconium oxide precursor, a zirconium oxide precursor obtained by the process, and a catalyst for removing nitrogen oxides which comprises the zirconium oxide precursor.
4) Zirconium oxide obtained by dissolving a zirconium compound in an alcohol, allowing the zirconium compound to react at a temperature of from 0xc2x0 C. up to the boiling point of the system to precipitate a zirconium oxide precursor, and heat treating the resulting zirconium oxide precursor at a temperature ranging from 250 to 1500xc2x0 C., and a catalyst for decomposing nitrogen oxides which comprises the zirconium oxide.
5) A zirconium oxide composite obtained by dissolving a zirconium compound in an alcohol, allowing the zirconium compound to react at a temperature of from 0xc2x0 C. up to the boiling point of the system to precipitate a zirconium oxide precursor on a substrate immersed in the solution, and heat treating the precursor at a temperature ranging from 250 to 1500xc2x0 C.
6) A process for producing zirconium oxide having a cubic crystal structure which comprises precipitating crystals from an alcohol solution of a zirconium compound and firing the crystals, zirconium oxide having a cubic crystal structure obtained by the process, and a catalyst for decomposing nitrogen oxides which comprises the zirconium oxide having a cubic crystal structure.
7) A process for producing zirconium oxide having a monoclinic crystal structure which comprises precipitating crystals from an alcohol solution of a zirconium compound, aging the crystals in a controlled atmosphere, and firing the aged crystals, zirconium oxide having a monoclinic crystal structure obtained by the process, and a catalyst for decomposing nitrogen oxides which comprises the zirconium oxide having a monoclinic crystal structure.
According to the process for producing zirconium oxide of the present invention, zirconium oxide having a desired crystal structure can be produced easily and efficiently without involving a complicated step.
According to the process for producing zirconium oxide or a precursor thereof of the present invention, a thin and dense film comprising small particles of zirconium oxide or a precursor thereof can easily be formed on various substrates such as an organic substance, large-area substrates, or substrates of complicated surface profile such as a porous material by means of simple equipment. In using a porous film or a porous material as a substrate, zirconium oxide or a precursor thereof easily fills the inside of the pores on the substrate surface.
The zirconium oxide or the zirconium oxide precursor of the present invention is capable of effectively reducing or decomposing nitrogen oxides present in exhaust gas, etc. and therefore useful as a catalyst for removing or decomposing nitrogen oxides.